Mutagen-sensitive mutants defective in DNA repair mechanisms have been collected in Drosophila melanogaster, and characterized cytogenetically in order to gain a basic understanding of the genetic control of sensitivity to mutagenic agents. Most of our recent work has centered on the mutants for the mei-41 gene. These mutants are sensitive to a wide variety of mutagenic agents, defective in postreplication repair and meiotic recombination, female sterile, and have fragile chromosomes. The mei-41 gene is a hot spot for EMS and P-element insertion mutagenesis, and shows a high frequency of interallelic meiotic recombination, suggesting that the gene is physically large. To confirm this hypothesis and to better understand the structure and regulation of the gene, it has been cloned and is being characterized molecularly. The mei-41 gene controls the activity of a specific DNA endo-exonuclease that is antigenically related to endo-exonucleases from yeast and Neurospora. A DNA sequence that codes for the nuclease in yeast has been hybridized in situ to Drosophila salivary gland chromosomes. A single site of hybridization has been identified in region 56D on chromosome 2; mei-41, however, is located in region 14C on the X chromosome. We are currently looking for second chromosome mutants that lack the nuclease activity, and cloning the nuclease gene using the yeast sequence as a probe.